What's New on the Moon?

by Dr. Bevan M. French

In 1969 over a billion people witnessed the "impossible" coming
true as the first men walked on the surface of the Moon. For the next
three years, people of many nationalities watched as one of the great
explorations of human history was displayed on their television
screens.

Between 1969 and 1972, supported by thousands of scientists and
engineers back on Earth, 12 astronauts explored the surface of the
Moon. Protected against the airlessness and the killing heat of the
lunar environment, they stayed on the Moon for days and some of them
travelled for miles across its surface in Lunar Rovers. They made
scientific observations and set up instruments to probe the interior
of the Moon. They collected hundreds of pounds of lunar rock and soil,
thus beginning the first attempt to decipher the origin and geological
history of another world from actual samples of its crust.Image from NASA Spacelink
The initial excitement of new success and discovery has passed.
The TV sets no longer show astronauts moving across the sunlit lunar
landscape. But here on Earth, scientists are only now beginning to
understand the immense treasure of new knowledge returned by the
Apollo astronauts.

The Apollo Program has left us with a large and priceless legacy
of lunar materials and data. We now have Moon rocks collected from
eight different places on the Moon. The six Apollo landings returned
a collection weighing 382 kilograms (843 pounds) and consisting of
more than 2,000 separate samples. Two automated Soviet spacecraft
named Luna-16 and Luna-20 returned small but important samples
totalling about 130 grams (five ounces).

Instruments placed on the Moon by the Apollo astronauts as long
ago as 1969 are still detecting moonquakes and meteorite impacts,
measuring the Moon's motions, and recording the heat flowing out from
inside the Moon. The Apollo Program also carried out a major effort of
photographing and analyzing the surface of the Moon. Cameras on the
Apollo spacecraft obtained so many accurate photographs that we now
have better maps of parts of the Moon than we do for some areas on
Earth. Special detectors near the cameras measured the weak X-rays and
radioactivity given off by the lunar surface. From these measurements,
we have been able to determine the chemical composition of about
one-quarter of the Moon's surface, an area the size of the United
States and Mexico combined. By comparing the flight data with analyses
of returned Moon rocks, we can draw conclusions about the chemical
composition and nature of the entire Moon.

Thus, in less than a decade, science and the Apollo Program have
changed our Moon from an unknown and unreachable object into a
familiar world.

What Has the Apollo Program Told Us About the Moon?

What have we gained from all this exploration? Before the landing
of Apollo 11 on July 20, 1969, the nature and origin of the Moon were
still mysteries. Now, as a result of the the Apollo Program, we can
answer questions that remained unsolved during centuries of
speculation and scientific study:

(1) Is There Life On The Moon?

Despite careful searching, neither living organisms nor fossil
life have been found in any lunar samples. The lunar rocks were so
barren of life that the quarantine period for returned astronauts was
dropped after the third Apollo landing.

(2) What Is The Moon Made Of?

The Moon is made of rocks. The Moon rocks are so much like Earth
rocks in their appearance that we can use the same terms to describe
both. The rocks are all IGNEOUS, which means that they formed by the
cooling of molten lava. (No sedimentary rocks, like limestone or
shale, which are deposited in water, have ever been found on the
Moon.).

The dark regions (called "maria") that form the features of "The
Man in the Moon" are low, level areas covered with layers of basalt
lava, a rock similar to the lavas that erupt from terrestrial
volcanoes in Hawaii, Iceland, and elsewhere. The light-colored parts
of the Moon (called "highlands") are higher, more rugged regions that
are older than the maria. These areas are made up of several different
kinds of rocks that cooled slowly deep within the Moon. Again using
terrestrial terms, we call these rocks gabbro, norite, and
anorthosite.

Despite these similarities, Moon rocks are basically different
and it is easy to tell them apart by analyzing their chemistry or by
examining them under a microscope. The most obvious difference is that
Moon rocks have no water at all, while almost all terrestrial rocks
contain at least a percent or two of water. The Moon rocks are
therefore very well-preserved, because they never were able to react
with water to form clay minerals or rust. A 3 1/2-billion-year-old
Moon rock looks fresher than water-bearing lava just erupted from a
terrestrial volcano.

(3) What Is The Inside Of The Moon Like?

Sensitive instruments placed on the lunar surface by the Apollo
astronauts are still recording the tiny vibrations caused by meteorite
impacts on the surface of the Moon and by small moonquakes deep within
it. These vibrations provide the data from which scientists determine
what the inside of the Moon is like.

About 3,000 moonquakes are detected each year. All of them are
very weak by terrestrial standards. The average moonquake releases
about as much energy as a firecracker, and the whole Moon releases
less than one-ten-billionth of the earthquake energy of the Earth. The
moonquakes occur about 600 to 800 kilometers (370-500 miles) deep
inside the Moon, much deeper than almost all the quakes on our own
planet. Certain kinds of moonquakes occur at about the same time every
month, suggesting that they are triggered by repeated tidal strains as
the Moon moves in its orbits around the Earth.

A picture of the inside of the Moon has slowly been put together
from the records of thousands of moonquakes, meteorite impacts, and
the deliberate impacts of discarded Apollo rocket stages onto the
Moon. The Moon is not uniform inside, but is divided into a series of
layers just as the Earth is, although the layers of the Earth and Moon
are different. The outermost part of the Moon is a crust about 60
kilometers (37 miles) thick, probably composed of calcium-and
aluminium-rich rocks like those found in the highlands. Beneath the
crust is a thick layer of denser rock (the mantle) which extends down
to more than 800 kilometers (500 miles).

The deep interior of the Moon is still unknown. The Moon may
contain a small iron core at its center, and there is some evidence
that the Moon may be hot and even partly molten inside.

(4) What Is The Moon's Surface Like?

Long before the Apollo Program scientists could see that the
Moon's surface was complex. Earth-based telescopes could distinguish
the level maria and the rugged highlands. We could recognize countless
circular craters, rugged mountain ranges, and deep winding canyons or
rilles.

Because of the Apollo explorations, we have now learned that all
these lunar landscapes are covered by a layer of fine broken-up powder
and rubble about 1 to 20 meters (3 to 60 feet) deep. This layer is
usually called the "lunar soil," although it contains no water or
organic material, and it is totally different from soils formed on
Earth by the action of wind, water, and life.

The lunar soil is something entirely new to scientists, for it
could only have been formed on the surface of an airless body like the
Moon. The soil has been built up over billions of years by the
continuous bombardment of the unprotected Moon by large and small
meteorites, most of which would have burned up if they had entered the
Earth's atmosphere.

These meteorites form craters when they hit the Moon. Tiny
particles of cosmic dust produce microscopic craters perhaps 1/1000 of
a millimeter (1/25,000 inch) across, while the rare impact of a large
body may blasts out a crater many kilometers, or miles, in diameter.
Each of these impacts shatters the solid rock, scatters material
around the crater, and stirs and mixes the soil. As a result, the
lunar soil is a well-mixed sample of a large area of the Moon, and
single samples of lunar soil have yielded rock fragments whose source
was hundreds of kilometers from the collection site.

However, the lunar soil is more than ground-up and reworked lunar
rock. It is the boundary layer between the Moon and outer space, and
it absorbs the matter and energy that strikes the Moon fro the Sun and
the rest of the universe. Tiny bits of cosmic dust and high-energy
atomic particles that would be stopped high in the Earth's protective
atmosphere rain continually onto the surface of the Moon.

(5) How Old Is The Moon?

Scientists now think that the solar system first came into being
as a huge, whirling, disk-shaped cloud of gas and dust. Gradually the
cloud collapsed inward. The central part became masssive and hot,
forming the Sun. Around the Sun, the dust formed small objects that
rapidly collected together to form the large planets and satellites
that we see today.

By carefully measuring the radioactive elements found in rocks,
scientists can determine how old the rocks are. Measurements on
meteorites indicate that the formation of the solar system occurred
4.6 billion years ago. There is chemical evidence in both lunar and
terrestrial rocks that the Earth and Moon also formed at that time.
However, the oldest known rocks on Earth are only 3.8 billion years
old, and scientists think that the older rocks have been destroyed by
the Earth's continuing volcanism, mountain-building, and erosion.

(6) What Is The History Of The Moon?

The first few hundred million years of the Moon's lifetime were
so violent that few traces of this time remain. Almost immediately
after the Moon formed, its outer part was completely melted to a depth
of several hundred kilometers. While this molten layer gradually
cooled and solidfied into different kinds of rocks, the Moon was
bombarded by huge asteroids and smaller bodies. Some of these
asteroids were the size of small states, like Rhode Island or
Delaware, and their collisions with the Moon created huge basins
hundreds of kilometers across.

The catastrophic bombardment died away about 4 billion years ago,
leaving the lunar highlands covered with huge overlapping craters and
a deep layer of shattered and broken rock. As the bombardment
subsided, heat produced by the decay of radioactive elements began to
melt the inside of the Moon at depths of about 200 kilometers (125
miles) below its surface. Then, for the next half billion years, from
about 3.8 to 3.1 billion years ago, great floods of lava rose from the
inside the Moon and poured out over its surface, filling in the large
impact basins to form the dark parts of the Moon that we see today.

As far as we know, the Moon has been quiet since the last lavas
erupted more than 3 billion years ago. Since then, the Moon's surface
has been altered only by rare large meteorite impacts and by atomic
particles from the Sun and the stars. The Moon has preserved featured
formed almost 4 billion year ago, and if men had landed on the Moon a
billion years ago, it would have looked very much as it does now. The
surface of the Moon now changes so slowly that the footprints left by
the Apollo astronauts will remain clear and sharp for millions of
years.

This preserved ancient history of the Moon is in sharp contrast
to the changing Earth. The Earth still behaves like a young planet.
Its internal heat is active, and volcanic eruptions and
mountain-building have gone on continuously as far back as we can
decipher the rocks. According to new geological theories, even the
present ocean basins are less than about 200 million years old, having
formed by the slow separation of huge moving plates that make up the
Earth's crust.

(7) Where Did The Moon Come From?

Before we explored the Moon, there were three main suggestions to
explain its existence: that it had formed near the Earth as a separate
body; that it had separated from the Earth; and that is had formed
somewhere else and been captured by the Earth.

Scientists still cannot decide among these three theories.

What Has the Moon Told Us About the Earth?

It might seem that the active, inhabited Earth has nothing in
common with the quiet, lifeless Moon. Nevertheless, the scientific
discoveries of the Apollo Program have provided a new and unexpected
look into the early history of our own planet. Scientists think that
all the planets formed in the same way, by the rapid accumulation of
small bodies into large ones about 4.6 billion years ago. The Moon's
rocks contain the traces of this process of planetary creation. The
same catastrophic impacts and widespread melting that we recognize on
the Moon must also have dominated the Earth during its early years,
and about 4 billion years ago the Earth may have looked much the same
as the Moon does now.

The two worlds then took different paths. The Moon became quiet
while the Earth continued to generate mountains, volcanoes, oceans, an
atmosphere, and life. The Moon preserved its ancient rocks, while the
Earth's older rocks were continually destroyed and recreated as
younger ones.

The Earth's oldest preserved rocks, 3.3 to 3.8 billion years old,
occur as small remnants in Greenland, Minnesota, and Africa. These
rocks are not like the lunar lava flows of the same age. The Earth's
most ancient rocks are granites and sediments, and they tell us that
the Earth already had mountain-building, running water, oceans, and
life at a time when the last lava flows were pouring out across the
Moon.

In the same way, all traces of any intense early bombardment of
the Earth have been destroyed. The record of later impacts remains,
however, in nearly 100 ancient impact structures that have been
recognized on the Earth in recent years. Some of these structures are
the deeply eroded remnants of craters as large as those of the Moon
and they give us a way to study on Earth the process that once
dominated both the Earth and Moon.

Lunar science is also making other contributions to the study of
the Earth. The new techniques developed to analyze lunar samples are
now being applied to terrestrial rocks. Chemical analyses can now be
made on samples weighing only 0.001 gram (3/100,000 ounce) and the
ages of terrestrial rocks can now be measured far more accurately than
before Apollo. These new techniques are already helping us to better
understand the origin of terrestrial volcanic rocks, to identify new
occurrences of the Earth's oldest rocks, and to probe further into the
origin of terrestrial life more than 3 billion years ago.

What Else Can the Moon Tell Us?

Although the Apollo Program officially ended in 1972, the active
study of the Moon goes on. More than 125 teams of scientists are
studying the returned lunar samples and analyzing the information that
continues to come from the instruments on the Moon. Less than 10
percent of the lunar sample material has yet been studied in detail,
and more results will emerge as new rocks and soil samples are
examined.

The scientific results of the Apollo Program have spread far
beyond the Moon itself. By studying the Moon, we have learned how to
go about the business of exploring other planets. The Apollo Program
proved that we could apply to another world the methods that we have
used to learn about the Earth. Now the knowledge gained from the Moon
is being used with the photographs returned by Mariner 9 and 10 to
understand the histories of Mercury and Mars and to interpret the data
returned by the Viking mission to Mars.

Note for Scientists and Educators

The Lunar Science Institute in Houston, Texas can provide further
information about lunar science and about data resources that are
available for scientific and educational purposes. In particular, the
Institute maintains lists of available books, articles, photographs,
maps, and other materials dealing with the Moon and the Apollo
missions. For further information, contact: